It is usual for such a respirator to have a filter formed on a canister which is attached to an air-inlet of the face-piece of the respirator. The canister is bulky and it is important that it provides minimum obstruction for the user. Thus depending on the "handedness" of the user the canister is attached on either the right hand side or the left hand side of the face-piece. However, such an arrangement has the problem that the flow of air in the face-piece is assymetric, because of the asymmetric position of the inlet. Prior art attempts have been made to overcome this problem by providing an air guide which, in use, lies between the face-piece and the face of the user. Such an air guide takes the form of an assymetric shield arranged so that air drawn through the inlet orifice(s) to the face-piece has to pass around the air guide to reach the face of the user. However, it has been found that the known air guides do not provide sufficient symmetry of flow, so that cold air drawn into the face-piece passes preferentially over one eye of the user rather than the other thereby affecting vision. An alternative in the prior art has involved the moulding of a closed duct into the thickness of the face-piece, something which presents great manufacturing problems.